Higher Education. Existing institutions are selling inefficient and grossly overpriced products in an environment in which objective evaluation is hard to obtain and the status quo fears innovation. If you could crack that nut, it would be massive.
There is another bigger one: College Book Industry. The current system is pretty inefficient. This probably differs from country to country. The current system here works like this: The universities give lists of subjects and prescribed textbooks to brick-and-mortar bookstores. They then buy the books and sell them at a huge mark-up. Students generally do not order online because of the time delay/import BS and many…
The lead time issue could be fixed simply by changing University policy, requiring professors to disclose required textbooks X weeks in advance of the first day of class.
One loophole I found near the end of undergrad was that the campus bookstore's website had the required books posted for each class well over a month ahead of time (so you could buy them online and swing by to pick them up). I just looked up all my classes there, copied down the ISBNs and hit up half.com or Amazon. Of course, this breaks down with last minute changes or additions made by professors.